It is conventional to couple a truck tractor to a leading or first trailer and to couple the leading trailer to a second or trailing trailer. The tractor has ground engageable wheels provided with brakes, the first trailer has ground engageable wheels provided with its own brakes, and the second trailer has ground engageable wheels provided with its own brakes. When the tractor and the two trailers are in condition for operation, all of the brakes should be operable from the tractor in response to the driver's actuation of a brake pedal. When the first trailer is uncoupled from the tractor and parked, the brakes of such trailer are applied via a spring assist mechanism. When the second trailer is uncoupled from the first trailer and parked, the brakes of the second trailer are applied via a similar spring assist mechanism. The brakes of the second trailer will continue to be applied until such trailer is coupled to the first trailer and the spring assist mechanism deactivated by means of pressurized fluid. To release the parking brakes of the second trailer and to operate the brakes of both trailers in unison such brakes must be coupled to a fluid (air) compressor which normally is positioned within the tractor.
It is essential that the brakes of both trailers be operable simultaneously so as to apply and release the brakes in unison. However, current brake systems utilize one valve for controlling the application and release of the brakes when the trailer is parked and a second valve for controlling the application and release of the brakes when the second trailer is in service. In some instances, one or the other valve is improperly set, thereby adversely affecting the performance of the second trailer's brakes. For example, if the appropriate valve associated with the parking brake control mechanism is not actuated to couple the spring assist brake mechanism to the source of pressure fluid, the parking brakes of the trailing trailer will not be released, thereby making it impossible for the assembled tractor and trailers to operate effectively.
In the event the spring assisted brake mechanism is uncoupled from the source of pressurized fluid during service operation of the vehicles, the brakes of the trailing trailer may remain in their released or inactive condition, thereby creating a hazardous condition wherein such trailer may be movable when it has been parked. Further, in the event the pedal actuated braking mechanism of the trailing trailer is not disconnected from the pressurized fluid source, it may not be possible to effect proper braking operation under the control of the driver.
Also typical for large vehicles, such as trucks pulling one or more trailers or load carrying attachments, are braking systems configured with two compressed air circuits. One of the compressed air circuits is usually called the “service circuit or line”. The other compressed air circuit is usually called the “emergency circuit or line”. The emergency circuit applies brakes and stops a truck if the service circuit fails. The brake system has pressure gauges that inform the driver of a failure of the emergency circuit. It is necessary in the case of a failure in either circuit to correct the failure. Air brake failures are not common on trucks driven on paved highways. Failures occur more often on trucks used on unpaved surfaces. Furthermore, multiple trailer configurations are often used to transport construction materials such as sand and gravel and agricultural products. Such activities often involve off road movement. Frequently the second trailer must be disconnected from the first trailer in such operations to facilitate unloading of the trailers.
A principal object of the apparatus disclosed herein is to provide an improved brake control system for use in connection with tractor-trailer combinations wherein there are at least two trailers coupled to one another in tandem.